PET was first developed in 1975 by two different groups of scientists. The first group is comprised of Dr. Zang-Hee Cho, et al. at University of California at Los Angeles (UCLA), while the second group is composed of Dr. M. Ter-Pogossian and Dr. M. Phelps, et al. at Washington University, St. Louis, Mo. Since then, PET has been further developed and innovated by several different commercial companies, including CPS-CTI. PET has been the only machine capable of performing molecular and functional imaging both on the body and the brain until 1992 (Although fMRI appeared in 1992, it was limited to the brain and the hemodynamics).
MRI, on the other hand, has been developed in 1973 by Dr. P. Lauterbur. It is somewhat similar to CT or PET, but is different in terms of physical principle. Over 10,000 MRI units are now in use at various hospitals throughout the world. MRI is essentially a morphological or anatomical imaging tool rather than functional, and thus lacks molecular specificity. However, MRI has much higher temporal and anatomical resolutions than PET. In 1992, a functional imaging capability has been incorporated into MRI by Dr. S. Ogawa, hence resulting in the creation of fMRI. By incorporating the use of such additional capability, fMRI became one of the most powerful brain imaging tools in the field of neuroscience.
When the fMRI was first introduced into the world, it was so impressive in brain imaging that the entire neuroscience community embraced this new device with great enthusiasm. The fMRI had indeed changed the landscape of neuroscience research. This excitement was short lived, however, as the demands for molecular specificity arose, which essentially renewed the interest in PET. As is well known in the art, PET has two major functional capabilities, namely, the functional capabilities for measuring metabolism of certain substrates such as glucose and ganciclovir and affinity/distributions of specific neuro-receptors for a certain ligand (i.e., molecular specificity and sensitivity). Theses capabilities are generally lacking in fMRI or MRI.
As explained above, PET and MRI are characterized by their own advantages and disadvantages. More specifically, PET is capable of providing molecular and functional information on human tissues with exceptionally high contrast. However, PET is limited in providing accurate anatomical information since it has inherently lower spatial resolution. Contrary to PET, however, MRI is capable of providing detailed anatomical information on human tissues, but cannot provide molecular and functional information.
Due to the foregoing pros and cons of PET and MRI, there have been many attempts in the art to integrate them together. However, none of the prior attempts achieved any practical success. For example, FIG. 1 shows a prior attempt for integrating conventional MRI (using 1.5-3.0T magnetic field) and PET (or PET/CT (Computer Tomography)). More specifically, a conventional system 100 is comprised of an MRI device 120 and a PET/CT device 130. As is well known in the art, the MRI device 120 measures atomic, chemical and physical aspects of a given tissue by using magnetic properties of subject materials that are present in the human body. As shown in FIG. 1, the MRI device 120 manipulates the measurements in order to produce an MRI image 122, which include anatomical information on human tissues. As is further well known in the art, the PET/CT 130 detects gamma rays (i.e., the 511 kev annihilation photons), which are used to produce a PET image 132 representing molecular and functional information on human tissues. The gamma rays originate from a biological sample that is marked by a positron-emitting radionuclide, such as F18, and are introduced into the human body. When a positron is emitted from the radionuclide and encounters an electron in the body, a pair of gamma rays is generated.
In such conventional system, the MRI device 120 and the PET/CT device 130 are totally separated from each other. They are placed distantly apart from each other and located in different spaces. The reason why the MRI device 120 and the PET/CT device 130 cannot be placed in close proximity of each other is due to the strong magnetic field generated by the MRI device 120, which can damage the PET/CT device 130. In particular, a photomultiplier used in the PET/CT device 130 is very sensitive to even a small external magnetic field. Therefore, the PET/CT device 130 cannot normally operate when the MRI device 120 is located in close proximity thereto.
In the conventional system, a patient has to be frequently transported in and out. This is because the patient has to be moved from a place, which is installed with the MRI device 120, to a different place where the PET/CT device 130 is located. A PET imaging is usually taken after an MRI imaging. However, an MRI imaging may precede a PET imaging. Therefore, even if the MRI and PET images are obtained, it is very difficult to combine them with a precision that is needed in image fusion. This is due to the physical separation between the MRI device 120 and the PET/CT device 130, especially when the desired resolution is high. Thus, there is a difficulty in combining a molecular image from the PET device 130 with an anatomical image from the MRI device 120, with an arrangement as shown, that is, when they are separated.
In addition, because the two images (i.e., one from MRI and the other from PET) are taken at different places (different environments or conditions) and times (metabolic changes will occur between them), it is highly possible that the conditions between such times and places may change and thus inconsistency is likely to be introduced. In other words, it is generally not suitable to combine an anatomical image from the MRI device 120 (or an oxygen consumption or hemodynamic image from fMRI) with a molecular image from the PET/CT device 130 in a conventional setting, especially in brain imaging due to the fine details of the brain structures.
Accordingly, there is a need for a system capable of providing a medical image that is truly integrated and contains both the anatomical information and molecular information within a time frame that is suitable for brain's functional changes or dynamics.